(1) Field of the Invention
This invention relates to protective barrier structures for electrical insulators, and more particularly to particular cover structures for shed-type insulators and their use with power transmission lines.
(2) Description of the Related Art
In the field of electrical power transmission, high voltage power lines (typically operating in excess of 1 kV) are supported by structures such as utility poles. To prevent leakage of power from the lines into the supporting structure, the lines are advantageously held at one end of an insulator, which in turn is held at its other end by the supporting structure. Common insulators are formed of ceramic, porcelain, epoxy, or other electrically nonconductive materials. As such insulators are used in the open, they are exposed to rain (including acid rain), humidity, salt fog, acid fog, particulate pollutants, and other environmental contaminants.
If a continuous surface of electroconductive moisture existed between the ends of the insulator (e.g., deposited as rain), it would provide a conductive path between the line and the supporting structure. To avoid this, insulators are configured so that water does not accumulate over a continuous surface between the two ends. In "shed"-type insulators this may involve providing the insulator as the combination of: a body (also known as a core or stem) which is formed in a generally circular cylindrical or frustoconical shape; and a number of "sheds" formed as annular flanges projecting radially outward and longitudinally/vertically downward from the body. Both the upper surface of each shed and the lower surface (underside) of each shed along substantial portions thereof are inclined downward, leaving the underside of each shed largely protected from falling rain and preventing a continuous flow of water from end-to-end.
The surfaces of such insulators may be contaminated in other ways so as to compromise their insulative properties. For example, in coastal areas, wind-blown, salt-laden, mist may deposit salt over substantially the entire exposed surface of the insulator. Such salt can become embedded in the porous surface of the insulator and, eventually, provide a conductive pathway from the wire to the supporting structure. Wind-blown sand may abrade the surface of the insulator, increasing porosity and rendering the insulator more susceptible to later water or salt contamination. In industrial areas, chemical and particulate pollution may similarly compromise the insulator. To partially address these problems, it is known to periodically wash the insulators with a high pressure stream of water. Such a stream may be delivered from the ground or from the air such as from a helicopter. To avoid the risk of conducting electricity from the lines through the stream of water, power to the lines is advantageously shut off during cleaning. Although shutting off the power for cleaning may be optional, it is substantially required during replacement of the insulator. Furthermore, cleaning may not be complete and the service life of the insulator may be diminished.
It is thus desirable to reduce the required cleaning of insulators and extend their service lives.